Publications (53)145.09 Total impact
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ABSTRACT: We perform equilibrium paralleltempering simulations of the 3D Ising EdwardsAnderson spin glass in a field. A traditional analysis shows no signs of a phase transition. Yet, we encounter dramatic fluctuations in the behaviour of the model: Averages over all the data only describe the behaviour of a small fraction of it. Therefore we develop a new approach to study the equilibrium behaviour of the system, by classifying the measurements as a function of a conditioning variate. We propose a finitesize scaling analysis based on the probability distribution function of the conditioning variate, which may accelerate the convergence to the thermodynamic limit. In this way, we find a nontrivial spectrum of behaviours, where a part of the measurements behaves as the average, while the majority of them shows signs of scale invariance. As a result, we can estimate the temperature interval where the phase transition in a field ought to lie, if it exists. Although this wouldbe critical regime is unreachable with present resources, the numerical challenge is finally well posed.  [Show abstract] [Hide abstract]
ABSTRACT: We study the offequilibrium dynamics of the threedimensional Ising spin glass in the presence of an external magnetic field. We have performed simulations both at fixed temperature and with an annealing protocol. Thanks to the Janus specialpurpose computer, based on fieldprogrammable gate array (FPGAs), we have been able to reach times equivalent to 0.01 s in experiments. We have studied the system relaxation both for high and for low temperatures, clearly identifying a dynamical transition point. This dynamical temperature is strictly positive and depends on the external applied magnetic field. We discuss different possibilities for the underlying physics, which include a thermodynamical spinglass transition, a modecoupling crossover, or an interpretation reminiscent of the random firstorder picture of structural glasses.  [Show abstract] [Hide abstract]
ABSTRACT: We report a highprecision finitesize scaling study of the critical behavior of the threedimensional Ising EdwardsAnderson model (the Ising spin glass). We have thermalized lattices up to L=40 using the Janus dedicated computer. Our analysis takes into account leadingorder corrections to scaling. We obtain Tc = 1.1019(29) for the critical temperature, \nu = 2.562(42) for the thermal exponent, \eta = 0.3900(36) for the anomalous dimension and \omega = 1.12(10) for the exponent of the leading corrections to scaling. Standard (hyper)scaling relations yield \alpha = 5.69(13), \beta = 0.782(10) and \gamma = 6.13(11). We also compute several universal quantities at Tc.  [Show abstract] [Hide abstract]
ABSTRACT: This paper describes the architecture, the development and the implementation of Janus II, a new generation applicationdriven number cruncher optimized for Monte Carlo simulations of spin systems (mainly spin glasses). This domain of computational physics is a recognized grand challenge of highperformance computing: the resources necessary to study in detail theoretical models that can make contact with experimental data are by far beyond those available using commodity computer systems. On the other hand, several specific features of the associated algorithms suggest that unconventional computer architectures, which can be implemented with available electronics technologies, may lead to order of magnitude increases in performance, reducing to acceptable values on human scales the time needed to carry out simulation campaigns that would take centuries on commercially available machines. Janus II is one such machine, recently developed and commissioned, that builds upon and improves on the successful JANUS machine, which has been used for physics since 2008 and is still in operation today. This paper describes in detail the motivations behind the project, the computational requirements, the architecture and the implementation of this new machine and compares its expected performances with those of currently available commercial systems. 
Article: Reconfigurable computing for Monte Carlo simulations: Results and prospects of the Janus project
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ABSTRACT: We describe Janus, a massively parallel FPGAbased computer optimized for the simulation of spin glasses, theoretical models for the behavior of glassy materials. FPGAs (as compared to GPUs or manycore processors) provide a complementary approach to massively parallel computing. In particular, our model problem is formulated in terms of binary variables, and floatingpoint operations can be (almost) completely avoided. The FPGA architecture allows us to run many independent threads with almost no latencies in memory access, thus updating up to 1024 spins per cycle. We describe Janus in detail and we summarize the physics results obtained in four years of operation of this machine; we discuss two types of physics applications: long simulations on very large systems (which try to mimic and provide understanding about the experimental nonequilibrium dynamics), and lowtemperature equilibrium simulations using an artificial parallel tempering dynamics. The time scale of our nonequilibrium simulations spans eleven orders of magnitude (from picoseconds to a tenth of a second). On the other hand, our equilibrium simulations are unprecedented both because of the low temperatures reached and for the large systems that we have brought to equilibrium. A finitetime scaling ansatz emerges from the detailed comparison of the two sets of simulations. Janus has made it possible to perform spinglass simulations that would take several decades on more conventional architectures. The paper ends with an assessment of the potential of possible future versions of the Janus architecture, based on stateoftheart technology.  [Show abstract] [Hide abstract]
ABSTRACT: Spin glasses are a longstanding model for the sluggish dynamics that appears at the glass transition. However, spin glasses differ from structural glasses for a crucial feature: they enjoy a time reversal symmetry. This symmetry can be broken by applying an external magnetic field, but embarrassingly little is known about the critical behaviour of a spin glass in a field. In this context, the space dimension is crucial. Simulations are easier to interpret in a large number of dimensions, but one must work below the upper critical dimension (i.e., in d<6) in order for results to have relevance for experiments. Here we show conclusive evidence for the presence of a phase transition in a fourdimensional spin glass in a field. Two ingredients were crucial for this achievement: massive numerical simulations were carried out on the Janus specialpurpose computer, and a new and powerful finitesize scaling method.  [Show abstract] [Hide abstract]
ABSTRACT: We numerically study the aging properties of the dynamical heterogeneities in the Ising spin glass. We find that a phase transition takes place during the aging process. Staticsdynamics correspondence implies that systems of finite size in equilibrium have static heterogeneities that obey FiniteSize Scaling, thus signaling an analogous phase transition in the thermodynamical limit. We compute the critical exponents and the transition point in the equilibrium setting, and use them to show that aging in dynamic heterogeneities can be described by a FiniteTime Scaling Ansatz, with potential implications for experimental work.  [Show abstract] [Hide abstract]
ABSTRACT: We present a massive equilibrium simulation of the threedimensional Ising spin glass at low temperatures. The Janus specialpurpose computer has allowed us to equilibrate, using parallel tempering, L=32 lattices down to T=0.64 Tc. We demonstrate the relevance of equilibrium finitesize simulations to understand experimental nonequilibrium spin glasses in the thermodynamical limit by establishing a timelength dictionary. We conclude that nonequilibrium experiments performed on a time scale of one hour can be matched with equilibrium results on L=110 lattices. A detailed investigation of the probability distribution functions of the spin and link overlap, as well as of their correlation functions, shows that Replica Symmetry Breaking is the appropriate theoretical framework for the physically relevant length scales. Besides, we improve over existing methodologies to ensure equilibration in parallel tempering simulations. Comment: 48 pages, 19 postscript figures, 9 tables. Version accepted for publication in the Journal of Statistical Mechanics  [Show abstract] [Hide abstract]
ABSTRACT: We study the 3D disordered Potts model with p = 5 and 6. Our numerical simulations (that severely slow down for increasing p) detect a very clear spin glass phase transition. We evaluate the critical exponents and the critical value of the temperature, and we use known results at lower p values to discuss how they evolve for increasing p. We do not find any sign of the presence of a transition to a ferromagnetic regime.  [Show abstract] [Hide abstract]
ABSTRACT: The out of equilibrium evolution of the EdwardsAnderson spin glass is followed for a tenth of a second, effectively halving the (logarithmic) temporal gap between previous simulations and experiments. In fact, we have been able to make safe predictions about the behavior at experimental times, using mild extrapolations. This work has been made possible by Janus, a special purpose computer designed by our collaboration. We have thoroughly studied the spin glass correlation functions and the growth of the coherence length for L 0 lattices in 3D,using L 4,40 lattices to check for finite size effects. We present clear evidence for a replicon correlator. Our main conclusion is that these spin glasses follow noncoarsening dynamics, at least up to the experimentally relevant time scales.  [Show abstract] [Hide abstract]
ABSTRACT: The out of equilibrium evolution for an EdwardsAnderson spin glass is followed for a tenth of a second, a long enough time to let us make safe predictions about the behaviour at experimental scales. This work has been made possible by Janus, an FPGA based special purpose computer. We have thoroughly studied the spin glass correlation functions and the growth of the coherence length for L = 80 lattices in 3D. Our main conclusion is that these spin glasses follow noncoarsening dynamics, at least up to the experimentally relevant time scales.  [Show abstract] [Hide abstract]
ABSTRACT: We perform numerical simulations, including parallel tempering, on the Potts glass model with binary random quenched couplings using the JANUS applicationoriented computer. We find and characterize a glassy transition, estimating the location of the transition and the value of the critical exponents. We show that there is no ferromagnetic transition in a large temperature range around the glassy critical temperature. We also compare our results with those obtained recently on the "random permutation" Potts glass. Comment: 7 pages and 3 figures. Corrected minor typos  [Show abstract] [Hide abstract]
ABSTRACT: We study numerically the nonequilibrium dynamics of the Ising spin glass, for a time spanning 11 orders of magnitude, thus approaching the experimentally relevant scale (i.e., seconds). We introduce novel analysis techniques to compute the coherence length in a modelindependent way. We present strong evidence for a replicon correlator and for overlap equivalence. The emerging picture is compatible with noncoarsening behavior. 
Conference Paper: Monte Carlo simulations for statistical physics: Janus
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ABSTRACT: Janus is an FPGAbased computer optimized for the simulation of spin glasses or similar condensed matter systems. Computing requirements in this area, are still riot met by available commercial systems, so all applicationdriven machine, boosting performance by approximately a factor 100x is in this case the only viable option to simulate large systems for a time window comparable with experiments.  [Show abstract] [Hide abstract]
ABSTRACT: We study numerically the nonequilibrium dynamics of the Ising Spin Glass, for a time that spans eleven orders of magnitude, thus approaching the experimentally relevant scale (i.e. {\em seconds}). We introduce novel analysis techniques that allow to compute the coherence length in a modelindependent way. Besides, we present strong evidence for a replicon correlator and for overlap equivalence. The emerging picture is compatible with noncoarsening behavior. 
Conference Paper: IANUS: Scientific computing on an FPGAbased architecture
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ABSTRACT: This paper describes the architecture and FPGAbased implementation of a massively parallel processing system (IANUS), carefully tailored to the computing requirements of a class of simulation problems relevant in statistical physics. We first discuss the system architecture in general and then focus on the configuration of the system for Monte Carlo simulation of spinglass systems. This is the first largescale application of the machine, on which IANUS achieves impressive performance. Our architecture uses largescale on chip parallelism (≃1000 computing cores on each processor) so it is a relevant example in the quickly expanding field of manycore architectures. 
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ABSTRACT: ETH Lab Eurotech Group, I33020 Amaro (Italy) This paper describes IANUS, a modular massively parallel and reconfigurable FPGAbased computing system. Each IANUS module has a computational core and a host. The computational core is a 4x4 array of FPGAbased processing elements with nearestneighbor data links. Processors are also directly connected to an I/O node attached to the IANUS host, a conventional PC. IANUS is tailored for, but not limited to, the requirements of a class of scientific applications characterized by regular code structure, large ratio of computation over database size and heavy use of bit manipulation operations. We assess the potential of this system with accurate performance figures of the IANUS prototype for a Monte Carlo code for statistical physics. For this application one module has the same performance as approximately 1500 highend PCs. Flexibility and reconfigurability 
Article: Ianus: an adaptive FPGA computer
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ABSTRACT: With Ianus, a nextgeneration fieldprogrammable gate array (FPGA)based machine, the authors hope to build a system that can fully exploit the performance potential of FPGA devices. A software platform that simplifies Ianus programming will extend its intended application range to a wide class of interesting and computationally demanding problems.  [Show abstract] [Hide abstract]
ABSTRACT: Without Abstract
Publication Stats
1k  Citations  
145.09  Total Impact Points  
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Institutions

19852014

Complutense University of Madrid
 • Department of Theoretical physics I
 • Facultad de Ciencias Físicas
 • Department of Theoretical physics II (Mathematical Methods of Physics)
Madrid, Madrid, Spain


2010

Sapienza University of Rome
 Department of Physics
Roma, Latium, Italy
